1. Field of the Invention
The present invention relates to a communication system and, more particularly, to digital radio communication technique for promoting the effective use of limited frequency resources.
2. Description of the Related Art
In a radio communication system made up of a central station and a plurality of peripheral stations, communication is sometimes effected by using both speech and packet data. For example, a very small aperture terminal (VSAT) satellite communication system having a central station and a plurality of peripheral stations, may be constructed such that mainly the central station and the peripheral stations interchange packet data in a star configuration, while peripheral stations perform speech communication with other peripheral stations in a mesh configuration or perform it with the central station in a star configuration.
In the system described above, for packet data communication in the star configuration, it is a common practice to assign a pair of carriers between the central station and a group of several to several hundred peripheral stations. The central station broadcasts multiplexed packet data to the peripheral stations by a continuous wave. Each peripheral station sends packetized data to the central station in bursts by a multiaccess system including random access or Time Division Multiple Access (TDMA).
On the other hand, for speech communication, a pair of carriers are assigned to the stations to effect the communication therebetween. In this case, the carriers are, in many cases, not fixedly assigned, but are assigned by demand assignment control. Specifically, the central station has a demand assignment control device. Each peripheral station communicates, every time a call is originated, with the demand assignment control device to request it to assign a carrier. After the carrier assignment, the peripheral station holds speech communication with the desired remote station. When the call ends, the carrier assigned to the peripheral station is released.
It has been customary with the above-stated system to prepare an exclusive carrier as a control channel for the communication of the peripheral station with the demand assignment control device at the beginning and end of a call. While a call is not originated, each peripheral station remains in a stand-by state, i.e., ready to send and receive the carrier for the control channel. On the origination of a call, the peripheral station communicates with the demand assignment control device over the control channel and, after carrier assignment, holds speech communication with the desired station by use of the assigned carrier. At the end of the call, the peripheral station restores the stand-by state while the demand assignment control device releases the carrier occupied for the speech communication.
The conventional exclusive carrier scheme has some problems left unsolved, as follows. Usually, the frequency at which call origination occurs is extremely low. Hence, fixedly assigning an exclusive carrier to the control channel is wasteful, considering limited frequency resources. In light of this, a plurality of stations have customarily shared a single control channel. As a result, there is needed a complicated control channel access circuit for executing random access, TDMA or similar access.
On the other hand, a communication system which assigns the same channel to a packet data signal and a speech signal and switches this channel is described in, for example, Japanese Laid-Open Patent Publication No. 105533/1988 (JP-A-63-105533). This system, however, needs to switch the same channel to the packet data signal and the speech signal.
Moreover, a technique of transmitting a request for a assignment of a speech channel in the form of a packet data signal and of communicating a speech signal by using the same as the channel of the packet data signal is described in, for example, Japanese Laid-Open Patent Publication Nos. 93946/1989 (JP-A-01-93946) or 98399/1989 (JP-A-01-98399). In this technique, however, the central station needs to be able to control a communication channel and switch speech data because the communication is done via the central station when one peripheral station communicates with another peripheral station.